Some one in the restoration/A12 section brought up a point about Hemi combustion chambers being more detonation resistant and was wondering what everyones thoughts on this theory are.

I read Bill Weertman's book on the History of Chrysler Engines. He was discussing the early theories regarding open chamber combustion chambers including the Hemi and how the thought was that they were more prone to detonation due to the lack of squish (turbulence). He goes on to say that their tests, however, showed that because of the surface area to volume ratio of the hemi and the spark plug location, the squish (and detonation problem) was negated.

I read another article on detonation where they again mentioned the Chrysler Hemi. In this article they state that while the lower compression pistons in the 2G Hemi could run reasonable timing, the high compression pistons needed to run considerably more total timing to make up for the the flame front disruption caused by the domes on the piston. Severe detonation could occur unless fuel with the proper octane was used.

Now, in my case, I have aluminum heads, 9.5 compression running on 94 octane fuel. With 34 degrees of total timing, curved with the timing all in by 2000 rpm, 3.54 gears, a 4 speed in a 3800 lb car, and running at 180 degree temperature, I can get the engine to ping slightly by dropping the rpm down to 2000 rpm in 3rd or 4th gear and winging the throttle quickly. I corrected this by changing the dizzy springs so the curve is all in by 2600 RPM. I can get a quick little ping going up 5% grade in 4th gear going about 50 MPH (2300 rpm), and open up the throttle a little.

Gentlemen (and ladies) start ur discussion!

Is your compression actual? What I'm asking is, are your pistons 9.5:1 or is your ACTUAL compression 9.5:1?

what octane fuel?

a hemispherical chamber is more detonation resistant. piston quench area can be achieved on a hemi just like a wedge.

A hemi has the spark plug in the center. That's good. It has a long way for flame travel. That's bad. It is possible to get squish in a Hemi by running the piston close to the head surface farther away from the spark plug. That's good. Most Hemis have combustion chambers flattened out over the top of the piston. That's bad. The supposition that a hemi has the least surface-to-volume ratio is only true if the combustion chamber looks like a hemisphere. If it looks like part of an orange peel it has a HIGH surface to volume ratio.

Look at www.theoldone.com to see a good hemispherical combustion chamber developed for a ford Boss 429.

R.

no comparison. boss 429 has a much much better chamber then the hemi. but kudos for the link.

Forget your compression ratio, what's the cylinder pressure? I'm guessing you're running a fairly small cam with a short duration figure. That will build cylinder pressure. My old stock 426 was a measured 9.56-1, with the stock cam it would ping under load too with standard grade gasoline. Dave

On timing, I believe the rule of thumb when pushing compression on pump gas is to bring it all in later - at 3000 RPM. Anyhow that's what I did on a 10.2 cr wedge with no problems (5 speed). I can accelerate from 1500 RPM in OD if I want without a ping.

I think spark plug location is the key.
Harley Sportsters prior to 1986 had hemi heads, but with the plug on one side, and very high piston domes. Factory timing spec was 45 degrees advance. And with only 9:1 compression, they needed a lot of octane (although they admittedly had primitive ignition without any vac advance).

Shupe

STUPE:
By having the ideal squish over a portion of the chamber, but leave enough volume at the TOP of the dome (and around the valves) to get the ratio you want.
R.